[ospfd] Fix multicast membership drop bug

[ospfd] Fix multicast membership drop bug

2006-06-15 Paul Jakma <paul.jakma@sun.com>

	* Reported by Milan Koci
	* ospf_interface.h: (struct ospf_if_info) Add reference counts
	  for multicast group memberships. Add various macros to help
	  manipulate/check membership state.
	* ospf_interface.c: (ospf_if_set_multicast) Maintain the
	  ospf_if_info reference counts, and only actually drop
	  memberships if it hits 0, to avoid losing membership when
	  OSPF is disabled on an interface with multiple active OSPF
	  interfaces.
	* ospf_packet.c: (ospf_{hello,read}) Use the new macros to
	  check/set
	  multicast membership.
	* ospf_vty.c: (show_ip_ospf_interface_sub) ditto.